Solid electrolytic capacitor and method of manufacturing the same

ABSTRACT

A solid electrolytic capacitor includes a positive electrode foil made of metal, a dielectric oxide layer provided on a surface of the positive electrode foil, a separator provided on the dielectric oxide layer, a solid electrolyte layer made of conductive polymer impregnated in the separator, a negative electrode foil facing the dielectric oxide layer across the solid electrolyte layer, and a phosphate provided on the dielectric oxide layer. This solid electrolytic capacitor reduces a leakage current.

TECHNICAL FIELD

The present invention relates to a solid electrolytic capacitor and amethod of manufacturing the capacitor.

BACKGROUND ART

As electronic devices operate at higher speeds at higher frequencies,electrolytic capacitors used in power supply lines of CPUs are requiredto have noise reduction performance and quick transient response overwide bandwidths from a low frequency to a high frequency of about 1 MHzto 1 GHz, and to have large capacitances and low impedance.

Roll type solid electrolytic capacitors that easily provide largercapacitance than capacitors including plural laminated electrode foilshave been put in the market. Roll type solid electrolytic capacitors aremade by winding a positive electrode foil and a negative electrode foillaminated with a separator in between. Such solid electrolyticcapacitors have superior high-frequency characteristics in addition tolong life time and temperature characteristics, and accordingly, arewidely adopted in power supply circuits of personal computers.

FIG. 4 is a partially cutaway perspective view of a conventionalelectrolytic capacitor 501 described in Patent Documents 1 and 2.Positive electrode foil 101 made of aluminum foil has a surfaceroughened by etching. A dielectric oxide layer is formed by anodizingthe surface. Negative electrode foil 102 is made of aluminum foil.Capacitor element 105 is formed by winding positive electrode foil 101and negative electrode foil 102 with insulating separator 103 inbetween.

Solid electrolyte layer 104 is made of conductive polymer impregnated inseparator 103. Positive electrode lead wire 106 and negative electrodelead wire 107 are joined to positive electrode foil 101 and positiveelectrode foil 102, respectively, and are drawn to outside. Case 108accommodates capacitor element 105. Case 108 is made of aluminum and hasa cylindrical shape having a bottom. Sealing member 109 made of resinvulcanized butyl rubber has holes 109A and 109B which positive electrodelead wire 106 and negative electrode lead wire 107 pass through,respectively, and seals an opening of case 108.

Upon a voltage being applied to, solid electrolytic capacitor 501 maycause a leakage current generating heat, hence shortening its life time.

Patent Document 1: JP10-340829A

Patent Document 2: JP2007-103499

SUMMARY OF THE INVENTION

A solid electrolytic capacitor includes a positive electrode foil madeof metal, a dielectric oxide layer provided on a surface of the positiveelectrode foil, a separator provided on the dielectric oxide layer, asolid electrolyte layer made of conductive polymer impregnated in theseparator, a negative electrode foil facing the dielectric oxide layeracross the solid electrolyte layer, and a phosphate provided on thedielectric oxide layer.

This solid electrolytic capacitor reduces a leakage current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway perspective view of a solid electrolyticcapacitor in accordance with an exemplary embodiment of the presentinvention.

FIG. 2 is a sectional view of the solid electrolytic capacitor inaccordance with the embodiment.

FIG. 3A shows evaluation results of the solid electrolytic capacitor inaccordance with the embodiment.

FIG. 3B shows evaluation results of the solid electrolytic capacitor inaccordance with the embodiment.

FIG. 4 is a partially cutaway perspective view of a conventional solidelectrolytic capacitor.

REFERENCE NUMERALS

-   1 Positive Electrode Foil-   1A Surface-   1B Dielectric Oxide Layer-   1C Electrolyte-   2 Negative Electrode Foil-   3 Separator-   4 Solid Electrolyte Layer-   4A Conductive Polymer

DETAIL DESCRIPTION OF PREFERRED EMBODIMENT

FIGS. 1 and 2 are partially cutaway perspective view and sectional viewof solid electrolytic capacitor 1001 according to an exemplaryembodiment of the present invention, respectively. Positive electrodefoil 1 made of metal, such as aluminum, has surface 1A roughened byetching. Dielectric oxide layer 1B is provided on surface 1A byanodizing surface 1A. Separator 3 having an insulating property isprovided on dielectric oxide layer 1B. Negative electrode foil 2 made ofmetal, such as aluminum, is provided on separator 3. That is, negativeelectrode foil 2 faces dielectric oxide layer 1B across separator 3.Positive electrode foil 1, dielectric oxide layer 1B, separator 3, andnegative electrode foil 2 are thus stacked and wound, providingcapacitor element 5. Conductive polymer 4A impregnated in separator 3constitutes solid electrolyte layer 4. That is, negative electrode foil2 faces dielectric oxide layer 1B across solid electrolyte layer 4.Electrolyte 1C made of phosphate is provided on dielectric oxide layer1B. Positive electrode lead wire 6 and negative electrode lead wire 7are joined with positive electrode foil 1 and negative electrode foil 2,respectively. Case 8 made of aluminum has bottom 8A, and has acylindrical shape having opening 8B. Case 8 accommodates capacitorelement 5 therein such that lead wires 6 and 7 are drawn out fromopening 8B. Sealing member 9 made of resin galvanized butyl rubber hasholes 9A and 9B which lead wires 6 and 7 pass through, respectively, andseals opening 8B of case 8.

A method of manufacturing solid electrolytic capacitor 1001 will bedescribed below.

First, dielectric oxide layer 1B is formed on surface 1A by anodizingsurface 1A of positive electrode foil 1. Positive electrode lead wire 6is joined to positive electrode foil 1. Negative electrode lead wire 7is joined to negative electrode foil 2. Then, positive electrode foil 1,separator 3, and negative electrode foil 2 are stacked such thatdielectric oxide layer 1B and negative electrode foil 2 face each otheracross separator 3 in between, and are wound with a winding machine,providing capacitor element 6. When stacked positive electrode foil 1,separator 3, and negative electrode foil 2 are wound, a crack or damagemay be caused on dielectric oxide layer 1B.

Then, while capacitor element 5 is immersed in an anodizing solution, avoltage is applied between lead wires 6 and 7 to form a dielectric oxidelayer on an end surface of positive electrode foil 1 by anodizing aportion of surface 1A exposed due to the crack or damage of dielectricoxide layer 1B. The anodizing solution contains phosphate aselectrolyte. According to this embodiment, the anodizing solutioncontains ammonium dihydrogen phosphate as the electrolyte. Then,capacitor element 5 is dried by heating.

Then, capacitor element 5 is immersed into a polymerizing solution toimpregnate conductive polymer 4A into separator 3, providing solidelectrolyte layer 4.

Then, capacitor element 5 is accommodated in case 8, and sealing member9 is placed in opening 8B to seal opening 8B by shrinking the case,thereby providing solid electrolytic capacitor 1001.

In a conventional method of manufacturing a solid electrolyticcapacitor, capacitor element 5 is immersed in the anodizing solution torestore the crack or damage of dielectric oxide layer 1B, and then, theanodizing solution is removed from capacitor element 5 by rinsingelement with water or cleaning fluid, and capacitor element 5 is dried.In the method of manufacturing solid electrolytic capacitor 1001 inaccordance with the embodiment, after element 5 is immersed in theanodizing solution to restore the crack or damage of dielectric oxidelayer 1B, capacitor element 5 is dried without the rinsing to causeelectrolyte 1C (phosphate) in the anodizing solution to remain ondielectric oxide layer 1B.

Thus, this method can eliminate the process of rinsing capacitor element5 of solid electrolytic capacitor 1001. Even after solid electrolytelayer 4 is formed, a crack or damage of dielectric oxide layer 1B may becaused due to physical stress applied to capacitor element 5. A voltageis applied to lead wires 6 and 7 after solid electrolytic capacitor 1001is completed, the phosphate, electrolyte, that remains on surface 1A ofpositive electrode foil 1 anodizes a portion of surface 1A of positiveelectrode foil 1 which is exposed due to the crack or damage ondielectric oxide layer 1B, and forms a dielectric oxide layer andrestores the crack or damage occurring on dielectric oxide layer 1B.This operation reduces a leakage current caused by the crack or damageon dielectric oxide layer 1B, thus providing the solid electrolyticcapacitor with high reliability.

According to this embodiment, capacitor element 5 includes positiveelectrode foil 1, negative electrode foil 2, and separator 3 that arewound. The solid electrolytic capacitor in accordance with theembodiment can include a capacitor element including positive electrodefoil 1 and negative electrode foil 2 facing each other across separator3 in between while positive electrode foil 1, negative electrode foil 2,and separator 3 are not wound.

Both of positive electrode foil 1 and negative electrode foil 2 of solidelectrolytic capacitor 1001 are made of aluminum. Negative electrodefoil 2 of capacitor element 5 in accordance with the embodimentinvention can be made of conductive material, such as nickel, titanium,or carbon, providing the same effects.

The electrolyte in the anodizing solution in accordance with theembodiment is phosphate, and is preferably ammonium dihydrogenphosphate. This anodizing solution has a high resistance to water and isalso used to form oxide layer 1B on surface 1A of positive electrodefoil 1. Therefore, this solution does will not affect solid electrolytelayer 4 of the completed solid electrolytic capacitor 1001.

In capacitor element 5, phosphate remains preferably by the amountranging from 0.5 μg to 50 μg per 1 cm² of positive electrode foil 1. Ifthe amount of the remaining phosphate is smaller than 0.5 μg, theremaining phosphate cannot anodize surface 1A of positive electrode foil1 sufficiently, thus preventing dielectric oxide layer 1B from beingrestored sufficiently. If the amount of the remaining phosphate isgreater than 50 μg, the remaining phosphate may disturb thepolymerization of conductive polymer 4A of solid electrolyte layer 4,thus preventing solid electrolyte layer 4 from being formed preferably.

In the case that the phosphate of the electrolyte in the anodizingsolution is ammonium dihydrogen phosphate, the amount of the phosphateremaining in capacitor element 5 is measured by the following method.First, the relationship between the concentration of the anodizingsolution in aqueous solution containing the anodizing solution and anelectrical conductivity of the aqueous solution is previously measuredto prepare a calibration curve showing the relationship between theelectrical conductivity and the concentration of the aqueous solution ofthe anodizing solution. Capacitor element 5 is disassembled after dried.Then, positive electrode foil 1 is immersed in a predetermined amount ofwater so as to dissolve of the anodizing solution attached to foil 1 inthe water, thereby preparing an extraction solution of the anodizingsolution.

Then, the electrical conductivity of the extraction solution is measuredto find the concentration of the anodizing solution in the extractionsolution based on the calibration curve. Then, the amount of thephosphate is calculated based on the concentration of the phosphate inanodizing solution and the volume of the extraction solution.

Then, the amount of phosphate per unit area of positive electrode foil 1is calculated based on the calculated amount of phosphate and the areaof positive electrode foil 1.

A sample of Example 1 of the solid electrolytic capacitor in accordancewith the embodiment was prepared. First, dielectric oxide layer 1B wasformed on surface 1A of positive electrode foil 1 by anodizing thesurface with an anodizing voltage of 12V. Then, capacitor element 1 wasprepared by the above method. Then, in order to restore a crack ordamage of dielectric oxide layer 1B inside capacitor element 1, whilecapacitor element 1 was immersed in 1.0 wt. % of an aqueous solution ofammonium dihydrogen phosphate at a temperature of 70° C., a dielectricoxide layer was formed by applying a voltage, such as 11V, not exceedingthe voltage (12V) of the anodizing between lead wires 6 and 7. Then,capacitor element 1 was taken out from the aqueous solution of ammoniumdihydrogen phosphate and dried by heating at a temperature ranging from100° C. to 180° C. for about 30 minutes. Then, ethylene-dioxy-thiophene(EDOT) or EDOT solution was impregnated in capacitor element 1. Then,butanol solution of ferric oxide of paratoluene sulphonic acid wasimpregnated in capacitor element 1. Then, capacitor element 1 was takenout from the butanol solution and heated at a temperature ranging from20° C. to 180° C. for not shorter than 30 minutes, thereby forming theconductive polymer on separator 3. Capacitor element 1 was thenaccommodated in case 8, and sealing member 9 was put into opening 8B ofthe case. Then, opening 8B of case 8 was sealed by a curling process.Capacitor element 1 was then subjected to an aging process in which a DCvoltage of 6.3V was applied between lead wires 6 and 7 at an ambienttemperature of 105° C. continuously for one hour, thereby preparing thesample of Example 1 of solid electrolytic capacitor 1 according to theembodiment having a diameter of 10 mm and a height of 10 mm. The ratedvoltage of the sample of example 1 was 6.3V.

A sample of Example 2 of solid electrolytic capacitor 1 having a ratedvoltage of 25V was manufactured by a method similar to the above method.For the sample of Example 2, the anodizing voltage for formingdielectric oxide layer 1B was 50V. The voltage for restoring dielectricoxide layer 1B after making capacitor element 1 was 48V. The voltage ofthe aging process after sealing case 8 was 25V.

Samples of Comparative Examples 1 and 2 with rated voltages of 6.3V and25V, respectively, were prepared by removing aqueous solution ofammonium dihydrogen phosphate by rinsing a capacitor element with waterfor 10 minutes after restoring the dielectric oxide layer. Further,samples of Comparative Examples 3 and 4 with rated voltages of 6.3V and25V, respectively, were prepared without restoring the dielectric oxidelayer.

Leakage currents were measured two minutes after applying a DC voltageof 6.3V to the samples of Examples 1 and 2 and Comparative Examples 1 to4. An equivalent series resistance (ESR) of each of the samples wasmeasured at 100 kHz. The amounts of ammonium dihydrogen phosphateremaining in the samples of Examples 1 and 2 were measured by the abovemethod. FIGS. 3A and 3B show measurement results of the leakage currentand the ESR of each of the samples of Examples 1 and 2 as a function ofthe amount of ammonium dihydrogen phosphate. FIG. 3B shows themeasurement results while enlarging the horizontal axis of FIG. 3A. Asshown in FIG. 3A, if the amount of ammonium dihydrogen phosphate islarger than 50 μg per 1 cm² of surface 1A of positive electrode foil 1,the leakage current and the ESR increase. As shown in FIG. 3B, if theamount of ammonium dihydrogen phosphate is smaller than 0.5 μg per 1 cm²of surface 1A of positive electrode foil 1, the leakage current alsoincreases. Thus, the phosphate remains by the amount ranging preferablyfrom 0.5 μg to 50 μg per 1 cm² of surface 1A of positive electrode foil1.

According to the embodiment, the conductive polymer constituting solidelectrolyte layer 4 is formed by polymerizing thiophene and itsderivative with oxidizing agent consisting of transition metal salt,thereby reducing the leakage current effectively

Solid electrolytic capacitor 1001 includes case 8 made of aluminumaccommodating capacitor element 5, but can include package resincovering capacitor element 5 instead of case 8.

INDUSTRIAL APPLICABILITY

A solid electrolytic capacitor according to the present inventionreduces a leakage current, and has high reliability, hence being usefulfor various electronic devices.

1. A solid electrolytic capacitor comprising: a positive electrode foilmade of metal and having a surface; a dielectric oxide layer provided onsaid surface of said positive electrode foil; a separator having aninsulating property and provided on said dielectric oxide layer; a solidelectrolyte layer made of conductive polymer impregnated in saidseparator; a negative electrode foil facing said dielectric oxide layeracross said solid electrolyte layer; and a phosphate provided on saiddielectric oxide layer.
 2. The solid electrolytic capacitor according toclaim 1, wherein said phosphate is ammonium dihydrogen phosphate.
 3. Thesolid electrolytic capacitor according to claim 1, wherein saidphosphate is provided by an amount ranging from of 0.5 μg to 50 μg per 1cm² of said positive electrode foil.
 4. The solid electrolytic capacitoraccording to claim 1, wherein said solid electrolyte layer containsconductive polymer formed by polymerizing thiophene and its derivativewith oxidizing agent consisting of transition metal salt.
 5. A method ofmanufacturing a solid electrolytic capacitor, comprising: forming adielectric oxide layer by anodizing a surface of a positive electrodefoil made of metal; stacking the positive electrode foil, a separatorhaving an insulating property, and a negative electrode foil made ofmetal such that the negative electrode foil faces the dielectric oxidelayer across the insulating separator; forming a capacitor element bywinding the positive electrode foil, the separator, and the negativeelectrode foil after said stacking the positive electrode foil, theseparator, and the negative electrode foil; restoring the dielectricoxide layer by immersing the capacitor element into an anodizingsolution containing electrolyte adapted to anodize the positiveelectrode foil; drying the capacitor element after said restoring thedielectric oxide layer so that the electrolyte remains on the positiveelectrode foil; and forming a solid electrolyte layer by impregnatingconductive polymer in the separator.
 6. The method according to claim 5,wherein the electrolyte is phosphate.
 7. The method according to claim6, wherein the electrolyte is ammonium dihydrogen phosphate.
 8. Themethod according to claim 5, wherein said drying the capacitor elementsuch that the electrolyte remains on the positive electrode foilcomprises drying the capacitor element so that the electrolyte remainson the surface of the positive electrode foil by an amount ranging from0.5 μg to 50 μg per 1 cm² of the surface of the positive electrode foil.9. The method according to claim 5, wherein said forming the solidelectrolyte layer comprises impregnating, into the separator, conductivepolymer obtained by polymerizing thiophene and its derivative withoxidizing agent consisting of transition metal salt.